1. Field of the Invention
This invention relates to data transmission systems, which include a plurality of equipment able to send and receive data frames using CSMA/CD media access control, and plural terminals having data frame sending and receiving functions, connected to the ports of the 5 hub units in a star shape in the data transmission systems. The invention further relates to control methods and multiplexing methods for such data transmission systems.
2. Description of the Related Art
The ISO 88802/3 Standard (IEEE8802/3 Standard) system represented by the Ethernet which is the mainstream of the current LAN performs a media access control method of CSMA/CD or Carrier Sense Multiple Access/Collision Detection. The CSMA/CD method monitors signals on the shared transmission media for all terminals (all equipment stations) and if there are no signals, the CSMA/CD method permits a data frame to be sent out on the media and if there are signals, on the trasmission media delays the sending of the data frame. Further, during the sending of a data frame , the CSMA/CD method detects if there is a collision with other terminals and when a collision has taken place, the CSMA/CD method suspends the sending the data frame, and after a prescribed time has elapsed, the CSMA/CD method resumes the sending the data frame. When the traffic load of the transmission media is increased, because of such operation of the CSMA/CD method, the collisions take place more frequently and the data frames cannot be transmitted for an indefinite time. The CSMA/CD method is therefore not suited for use in time-critical control systems.
In order to achieve the time-critical processing capable of exchanging certain information within a prescribed time window, the applicant previously proposed a CSMA/CD enhanced data transmission system, in which plural terminals which function to send and receive data frames are connected to ports of the hubs in the star shape (The Specification of Patent Application No. 51984/1996).
Shown in FIG. 10 is a system diagram illustrating the brief configuration of the previously applied data transmission system in which the main stay is a hub unit (described as a realtime hub unit) able to mutually exchange information within a certain prescribed time period by giving the right to use the transmission line to each terminal by adding new functions to the hub units which conform with the ISO8802/3 Standard.
That is, the star-shaped data transmission system shown in FIG. 10 is comprised of realtime hub units (hereinafter, simply referred to as hub units) 100, a plurality of terminals 50 having transmission circuits adapted to the ISO8802/3 Standard and transmission cables 40. The terminals 50 are connected to ports 1, 2, 3 . . . of each hub unit 100 by transmission cables 40. In the star-shaped data transmission system, the transmission approval with the priority to use the transmission cable is repetitively given to each terminal 50 conforming to the ISO8802/3 Standard, which is arranged in a star shape and connected to one of the ports of the hub units 100, by the transmission approval control function of the hub units 100. That is, transmission approval is given repetitively to each of the terminals 50, for instance, each station at a particular time or according to a predetermined order, some terminal 50 is given several times or some terminal 50 is skipped or some terminal 50 is given transmission approval once per several times. The transmission approval given to each terminal controls the number of frames that can be sent out at a transmission time, and the time that frames can be transmitted and thus, the transmission right can be obtained within a specified time period.
FIG. 11 illustrates the transmission approval control timings in the hub unit 100. In the transmission approval control of the hub unit, a preamble signal containing no data information (a dummy frame and PRE shown in FIG. 11) from the hub unit 100 is transmitted to all terminals 50 except one specific terminal and the transmission of a data frame (DT in FIG. 11) is made impossible for all terminals except the one specific terminal. The idle time between the preamble signal PRE and the data frame DT is the idle time between frames specified in the ISO8802/3 Standard (e.g, a value less than 9.8 .mu.sec. of ISO8802/3 Standard at 10 Mbps) so that a terminal does not move to the data frame sending operation by simply detecting no signal on the transmission line. When the data frame sending of a specific terminal is completed, the preamble signal PRE is sent to all terminals except another specific terminal and the right to use the transmission line is transferred to this specific terminal. This operation is repeated for every terminal in turn one at a time, in order according to a predetermined sequence, by jumping, or at a rate of once per several times and thus, all terminals are able to get the transmission right at regular intervals.
Further, by supervising a time in which each of the terminals 50 are able to send data at one time, an approved sending time is computed from a difference between the sending start time of the last time and the sending start time of a current sending time and an actual circulating time of the transmission right, the circulating time of the transmission right is made nearly constant by transferring the transmission right to a next terminal at the end point of the data frame DT sending time exceeding the approved sending time.
In the prior applied hub units 100 described above, to increase the number of terminals 50 or to distribute the terminals 50 in a wide range, it becomes necessary to make the hub unit 100 itself larger, the transmission cable 40 between the terminals 50 distributed in a wide range and the ports of the hub units 100 becomes long and signal attenuation must be compensated or wiring costs of the transmission cables 40 for eliminating noise effect to the transmission cables 40 will increase. In particular, when terminals are installed in each train car, signal lines must be wired extending over train cars, it is not desirable to concentrate signal lines to the hub units from the viewpoint of actual use.
That is, train cars may be separated when required and coupled to other train cars to compromise one train. It is therefore difficult to make the wiring of signal lines from train cars to concentrated hub units which correspond to the to flexibility of a train composition.
It is therefore desirable to provide a data transmission unit which functions like a single hub unit by dispersing plural hub units at required locations in a train car and consolidating them into one unit by connecting each to one another with the ports of the hub units and terminals connected by short signal lines.